Telescopic sights or riflescopes are used for precisely aiming firearms. To provide an accurate aiming indication, particularly for longer-range shots, it is important that the scope and its internal sighting reticle are properly positioned over the barrel of the firearm. The reticle, which typically includes at least one horizontal line and one vertical line, must be oriented relative to the barrel such that the horizontal crosshair is exactly level when the vertical crosshair is exactly vertically aligned with the axial center of the barrel bore. Because riflescopes are typically used for aiming at targets a sufficient distance away from the shooter that the projectile will drop by force of gravity an appreciable distance before reaching the target, if the reticle of the scope is not properly oriented relative to the barrel, the “drop” will appear to have a lateral component, causing the target to be missed or the expected amount of bullet drop to be miscalculated.
Modern riflescopes include a tube with optical lenses at each end and a reticle positioned generally at some midpoint there between. The reticle may be “zeroed” or adjusted vertically by an elevation adjustment mechanism and laterally by a windage adjustment mechanism. These adjustment mechanisms are generally mounted on an enlarged portion of the scope body known as the saddle. The bottom exterior surface of the saddle is often flat or includes a flat portion and the manufacture builds the scope with the reticle squared to this flat bottom surface at the factory. Thus, if this exterior bottom surface is positioned exactly horizontally when the scope is mounted to the firearm (i.e., exactly perpendicular to the vertical crosshair's alignment with the axis of the barrel bore), the internal reticle will be properly aligned.
Various methods have been used in the past for adjusting a rifle scope in mounting rings to assure proper alignment. Some methods use a bubble level or protractor-like apparatus that can be attached to the scope body during installation. Others have used a shim to fit between a known horizontal top surface of the firearm or mount to check the position. Each of these requires a separate device, precise measurements by the installer, or a shim of an exact size.